Metal Abundances and Star Formation Rates of Emission-line Galaxies in and around the Boötes Void

Abstract

We explore the possible dependencies of galaxy metal abundance and star formation rate (SFR) on local environment, focusing on the volume of space in and around the Boötes Void. Our sample of star-forming galaxies comes from the second catalog of the Hα-selected KPNO International Spectroscopic Survey (KISS), which overlaps the void. This sample represents a statistically complete, line-flux-limited ensemble of 820 star-forming galaxies, all of which possess metallicity and SFR estimates. We carry out two distinct analyses of the KISS galaxies: one that probes the properties of the entire sample as a function of local density, and a second that details the properties of 33 KISS star-forming galaxies located within the Boötes Void. In both cases, we find no evidence that either the metallicity of the KISS galaxies or their SFRs depend on the environments within which the galaxies are located. Our global analysis does show weak trends for decreasing stellar mass, decreasing metallicity, and decreasing SFRs with decreasing local densities. However, we argue that the metallicity and SFR trends are artifacts of the stellar mass—local density trend. In particular, the change in metallicity with density is precisely what one would predict from the mass–metallicity relation, given the observed drop in stellar mass with decreasing metallicity. Likewise, the SFR trend with density disappears when one instead considers the mass-normalized specific SFR. The KISS galaxies dwelling in the Boötes Void are found to have metallicity and SFR properties nearly identical to those of a matched comparison sample, despite the fact that the former are located in density environments that are, on average, more than 16 times lower.